Portable communication devices, e.g. mobile radio devices, in the meantime can enable communication in a multiplicity of different frequency bands and for a multiplicity of different transmission systems. To that end, they generally comprise a multiplicity of RF filters each provided for the corresponding frequency and the corresponding transmission system. Although modern RF filters in the meantime can be produced with small dimensions, on account of their multiplicity and the complexity of their interconnection the front-end modules in which the filters are arranged are nevertheless relatively large and their production is complex and expensive.
Tunable RF filters could remedy this. Such filters have a center frequency that is adjustable, for which reason a tunable filter, in principle, can replace two or more conventional filters. Tunable RF filters are known e.g. from the documents US 2012/0313731 A1 or EP 2530838 A1. In this case, the electroacoustic properties of resonators operating with acoustic waves are altered by tunable impedance elements.
The paper “Reconfigurable Multiband SAW Filters for LTE Applications”, IEEE SiRF 2013, pp. 153-155, by Lu et al., discloses filters that are reconfigurable by means of switches.
What is problematic about known tunable RF filters, however, is, in particular, the fact that the tuning itself alters important properties of the filters. In this regard, e.g. the insertion loss, the input impedance and/or the output impedance change(s) during tuning.